It is known that magnesium-containing zinc oxide, i.e., partially magnesium oxide solid-solubilized zinc oxide has a hexagonal Wurtzite crystal structure, and when an oxygen defect is introduced, emits bluish light under electron beam irradiation or Ultraviolet excitation (Non-Patent Document 1). Especially, magnesium-containing zinc oxide with a high concentration of solid-solubilized magnesium has a high luminance efficiency and can be used as an environmentally-benign zinc oxide blue phosphor, because zinc and magnesium from which it is made are abundant and nonpolluting resources.
Methods for producing such magnesium-containing zinc oxide include a solid reaction method, a coprecipitation method, and a hydrothermal synthesis method.
Among them, the solid reaction method, which reacts zinc oxide powder and magnesium oxide powder in their solid phase, can produce magnesium-containing zinc oxide at a low cost and in a large volume. However, because the solid solubility limit of magnesium in a solid-phase reaction is approximately 2 mol %, when magnesium is reacted at a ratio above a composition of Zn0.98Mg0.02O, this method results in deposition of cubic MgO, exhibiting a problem that magnesium cannot be solid-solubilized in zinc oxide at a high concentration.
One method for overcoming this problem is to cause a solid-phase reaction in the presence of a flux of potassium chloride or the like to allow magnesium solid-solubilize up to 20 mol % (Non-Patent Document 1). However, this method results in a large amount of impurities being contained in the product, and it is necessary but difficult to remove the flux components. Moreover, the reaction temperature required in the solid reaction method is a high temperature of 1300° C. or higher, which is another problem because zinc oxide might sublimate and make it difficult to obtain magnesium-containing zinc oxide having a uniform composition. Furthermore, in order to be used for a phosphor, the zinc oxide powder needs to have a high purity, a uniform particle size, and a good dispersibility. However, the solid reaction method results in production of an agglomerated particle state of zinc oxide, and requires additional steps of pulverization and classification in order to obtain a uniform particle size, which is yet another problem.
On the other hand, the coprecipitation method can produce magnesium-containing zinc oxide with a high concentration of solid-solubilized magnesium at a low temperature of 1000° C. or lower. For example, there is known a method of mixing a mixed aqueous solution of zinc and magnesium with an organic acid component to coprecipitate an organic acid double salt, and pyrolyzing the obtained organic acid double salt (Patent Document 1, Non-Patent Document 2).
The hydrothermal synthesis method can also produce magnesium-containing zinc oxide with a high concentration of solid-solubilized magnesium at a low temperature. For example, there is known a method of producing particles by forming a layer of magnesium-containing zinc oxide on micro-substrates made of zinc oxide, sapphire, or the like (Patent Document 2).